1. Field of the Invention
The present invention relates to the processing of radio frequency signals in a magnetic resonance imaging system and, particularly, to a method for processing radio frequency signals capable of transmitting radio frequency signals without a radio frequency signal transmitting channel provided by a coil plug on a patient bed, and to a radio frequency system for realizing the abovementioned radio frequency signal processing method.
2. Description of the Prior Art
A magnetic resonance imaging (MRI) system includes many types of coils, such as a body coil for covering an entire human body, a local coil for covering only a certain part of a human body, etc. The body coil is a self-transmitting and self-receiving coil. Generally speaking, since the coverage range of the body coil is larger, it needs a relatively high transmitting power, the signal-to-noise ratio of the images obtained is relatively low, and the signal-to-noise ratios at different positions in an image are not uniform. In case of the local coil, since its coverage range is relatively small, such as, for example, the knee region covered by a knee coil, the head region covered by a head coil, and the wrist region covered by a wrist coil, etc., the local coil receives only the radio frequency signals within a limited radio frequency excitation range (in order to distinguish them from the radio frequency signals in a transmitting phase, the radio frequency signals received by the coils are referred to as magnetic resonance signals hereafter), therefore, the signal-to-noise ratio of the image that is obtained is higher, and the signal-to-noise ratios at different positions in an image are relatively uniform.
For certain magnetic resonance systems in which a coil plug on a patient bed provides a radio frequency signal transmitting channel, a local coil connected to this type of coil plug can be designed as a self-transmitting and self-receiving coil. A local coil with the functions of a transmitting coil has a number of advantages in magnetic resonance applications, such as reducing the specific absorption rate (SAR), avoiding image fold-over problems due to imaging areas being too small, etc. In order to transmit the radio frequency signals from the coil plug and to send the received magnetic resonance signals via the coil plug to downstream devices such as an amplifier, such a self-transmitting and self-receiving local coil needs to have a switch for switching between transmitting and receiving modes, which is capable of undertaking high power radio frequency signals. The incorporation of the switch for switching between transmitting and receiving, however, not only makes the structure of the local coil complicated but also increases the cost of the local coil.
Furthermore, for those magnetic resonance imaging systems that do not provide any coil plug on the patient bed for radio frequency signal transmitting channels, since the abovementioned self-transmitting and self-receiving local coil cannot obtain from the coil plug the radio frequency signals for transmitting, the abovementioned self-transmitting and self-receiving coils cannot be used in this type of magnetic resonance imaging system, which can only use a local coil having only the receiving functions.
Therefore, it has become an urgent problem to be solved in the magnetic resonance imaging field regarding how to realize the transmission of radio frequency signals via a local coil without relying on a coil plug on a patient bed.